The present invention relates to power augmentation in gas turbine cycles and, more particularly, to an innovative method and system for efficiently combining steam and air injection for gas turbine power augmentation.
It is known that water or steam may be injected into a gas turbine at various points within a cycle to increase the mass flow of motive fluid to augment gas turbine power. However, the maximum amount of steam injection is limited. For example, there is a limit to the amount of steam which may be added into the combustion reaction zone in the liner because of degradation of the combustion process. Indeed, typically the total moisture entering the combustor is limited to 3.5-5% by weight of air flow, which limits the power augmentation capability with steam injection alone.
The use of an external motor driven compressor to inject air and increase gas turbine power output has also been proposed as described in U.S. Pat. Nos. 5,934,063 and 6,038,849. However, the motor power driving the air injection compressor becomes a significant auxiliary load in such systems.
A need remains for a method and system whereby gas turbine power output can be increased while eliminating the auxiliary power load of an air injection compressor.
The invention relates to a cycle in which steam is generated with the gas turbine exhaust energy, for example in a once through or drum boiler, at a pressure higher than required for steam injection, and in which this high pressure steam is expanded through a back pressure steam turbine which then drives an air injection compressor. The steam exhausted from the steam turbine can then be mixed with the air injection compressor discharge prior to injection into the combustion system of the gas turbine.
The cycle as proposed above thus eliminates the auxiliary motor load of an auxiliary air injection compressor, resulting in a cycle design with better power output and thermodynamic efficiency compared to a steam injection cycle, air injection cycle with motor driven compressor, or a combination thereof.
Accordingly, the invention is embodied in a gas turbine cycle comprising a gas turbine system including a compressor for compressing air, a combustion system for receiving compressed air from the compressor and a turbine for converting the energy of the combustion mixture into work; a heat recovery system for receiving exhaust gas from the gas turbine; a steam turbine; a first flow path for water and/or steam at elevated pressure including a first heat exchange flow path disposed in heat exchange relation to the exhaust gas flowing through the heat recovery system, thereby to produce superheated steam therein, and a superheated steam flow path for flowing the superheated steam to the steam turbine for converting the energy of the superheated steam into work; an auxiliary compressor operatively coupled to the steam turbine for being driven thereby to compress air to generate injection air at elevated pressure; and a flow path for the injection air, the injection air flow path being operatively coupled to at least one of the gas turbine compressor discharge and the combustion system of the gas turbine for increasing the mass flow of fluid thereinto. In an exemplary embodiment, there is also a flow path for steam exhausted from the steam turbine that is operatively coupled to at least one of the gas turbine compressor discharge and the combustion system of the gas turbine for increasing the mass flow of fluid thereinto.
In one embodiment, the injection air flow path includes a second heat exchange flow path disposed in heat exchange relation to the heat recovery system for heating the injection air.
As a further feature of one embodiment of the invention, the exhaust steam flow path is operatively coupled to the injection air flow path for mixing the exhaust steam with the injection air. Most preferably, the steam and air are mixed downstream of the second heat exchange flow path.
The invention is further embodied in a method of augmenting power generation with a gas turbine that comprises providing a gas turbine system including a compressor for compressing air, a combustion system for receiving compressed air from the compressor and a turbine for converting the energy of the combustion mixture that leaves the combustion system into work; heating at least one of water and steam at elevated pressure with exhaust gas from the gas turbine to produce superheated steam at elevated pressure; flowing said superheated steam to a steam turbine to convert the energy of the superheated steam into work; driving an auxiliary compressor with said steam turbine thereby to compress air to generate injection air at elevated pressure; and flowing said injection air to and injecting said injection air into at least one of the gas turbine compressor discharge and the combustion system of the gas turbine for increasing the mass flow of fluid thereinto.
In accordance with one embodiment of the invention, the method further comprises flowing steam exhausted from said steam turbine to at least one of the gas turbine compressor discharge and the combustion system of the gas turbine for increasing the mass flow of fluid thereinto. Advantageously, the exhaust steam is mixed with the injection air. Furthermore, the injection air may be heated with exhaust gas from the gas turbine, most preferably before it is mixed with the exhaust steam.